Assignment for lecture 1 (special senses)
Please fill in the blanks. In some cases I have given you two options, one of which is the correct
answer. Delete the one that does not apply or circle the one that does apply.
The eye can be seen as analogous to a camera, with the camera’s lens corresponding to the cornea and
the lens in the eye; of the two, the cornea has the greater refracting power. The refracting power of the
eye must be matched well to its length: if the eye is too long, the individual will suffer from myopia,
which can be corrected by adding a concave lens to decrease the total refracting power of the eye; of it
is too short, the individual will suffer from hyperopia, which can be corrected by adding a convex lens
to increase the total refracting power of the eye.
As we become older, it becomes more difficult to focus on near objects. Focusing on near objects
depends on increasing the refracting power of the eye, by changing the shape of the lens: to focus close
up, it must become more rounded. The process of changing refractive power to focus on near objects is
known as accommodation, and the deterioration on ageing is because the lens becomes stiffer due to
ageing of proteins.
The cells that detect light are found in the retina, the part of the eye that corresponds to the film of the
camera. These cells are known as photoreceptors, and are of two types: rods and cones. The rods are
mainly active in dim light, are not involved in colour vision and are found mainly in the peripheral area
of the retina. The cones are mainly active in bright light, are involved in colour vision and are found
mainly in the foveal area of the retina.
The ear can be divided into three sections: outer, middle and inner. The middle ear contains three tiny
bones known as the malleus, the incus and the stapes. Their function is to amplify sound, and this is
achieved by increasing the pressure. The bones connect the tympanic membrane, whose area is about
55 mm2, with the oval window, whose area is about 3.2 mm2, and this reduction in surface area is the
major mechanism underlying the pressure increase.
The part of the inner ear that is involved in hearing is known as the cochlea. The cells that convert
sound-induced vibration into electrical signals are known as hair cells and are of two types: inner and
outer. The inner are the main sensory cells. They form part of the organ of Corti, which sits on the
basilar membrane. Sound causes vibration of this membrane at specific points: near the oval window at
high frequencies, and far from the oval window at low frequencies.
The vestibular system maintains balance by allowing us to detect head position and movement. The
linear acceleration stimulus that we experience at all times results from the force of gravity, which
stimulates the otolith (or macula) organs. These organs are so named because they contain small
crystals of calcium carbonate, called “otoconia”. Head tilt thus stimulates these organs in the same way
as acceleration, with forward acceleration (e.g. being in a car starting from rest) being equivalent to
backward head tilt.
We can also detect angular acceleration, with the semicircular canals (of which we have 3 on each side,
at right angles to each other). These detect head turning, and are stimulated by turning in one direction,
while being inhibited by turning in the other direction.